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1perf-amd-ibs(1)
2===============
3
4NAME
5----
6perf-amd-ibs - Support for AMD Instruction-Based Sampling (IBS) with perf tool
7
8SYNOPSIS
9--------
10[verse]
11'perf record' -e ibs_op//
12'perf record' -e ibs_fetch//
13
14DESCRIPTION
15-----------
16
17Instruction-Based Sampling (IBS) provides precise Instruction Pointer (IP)
18profiling support on AMD platforms. IBS has two independent components: IBS
19Op and IBS Fetch. IBS Op sampling provides information about instruction
20execution (micro-op execution to be precise) with details like d-cache
21hit/miss, d-TLB hit/miss, cache miss latency, load/store data source, branch
22behavior etc. IBS Fetch sampling provides information about instruction fetch
23with details like i-cache hit/miss, i-TLB hit/miss, fetch latency etc. IBS is
24per-smt-thread i.e. each SMT hardware thread contains standalone IBS units.
25
26Both, IBS Op and IBS Fetch, are exposed as PMUs by Linux and can be exploited
27using the Linux perf utility. The following files will be created at boot time
28if IBS is supported by the hardware and kernel.
29
30 /sys/bus/event_source/devices/ibs_op/
31 /sys/bus/event_source/devices/ibs_fetch/
32
33IBS Op PMU supports two events: cycles and micro ops. IBS Fetch PMU supports
34one event: fetch ops.
35
36IBS PMUs do not have user/kernel filtering capability and thus it requires
37CAP_SYS_ADMIN or CAP_PERFMON privilege.
38
39IBS VS. REGULAR CORE PMU
40------------------------
41
42IBS gives samples with precise IP, i.e. the IP recorded with IBS sample has
43no skid. Whereas the IP recorded by regular core PMU will have some skid
44(sample was generated at IP X but perf would record it at IP X+n). Hence,
45regular core PMU might not help for profiling with instruction level
46precision. Further, IBS provides additional information about the sample in
47question. On the other hand, regular core PMU has it's own advantages like
48plethora of events, counting mode (less interference), up to 6 parallel
49counters, event grouping support, filtering capabilities etc.
50
51Three regular core PMU events are internally forwarded to IBS Op PMU when
52precise_ip attribute is set:
53
54 -e cpu-cycles:p becomes -e ibs_op//
55 -e r076:p becomes -e ibs_op//
56 -e r0C1:p becomes -e ibs_op/cnt_ctl=1/
57
58EXAMPLES
59--------
60
61IBS Op PMU
62~~~~~~~~~~
63
64System-wide profile, cycles event, sampling period: 100000
65
66 # perf record -e ibs_op// -c 100000 -a
67
68Per-cpu profile (cpu10), cycles event, sampling period: 100000
69
70 # perf record -e ibs_op// -c 100000 -C 10
71
72Per-cpu profile (cpu10), cycles event, sampling freq: 1000
73
74 # perf record -e ibs_op// -F 1000 -C 10
75
76System-wide profile, uOps event, sampling period: 100000
77
78 # perf record -e ibs_op/cnt_ctl=1/ -c 100000 -a
79
80Same command, but also capture IBS register raw dump along with perf sample:
81
82 # perf record -e ibs_op/cnt_ctl=1/ -c 100000 -a --raw-samples
83
84System-wide profile, uOps event, sampling period: 100000, L3MissOnly (Zen4 onward)
85
86 # perf record -e ibs_op/cnt_ctl=1,l3missonly=1/ -c 100000 -a
87
88Per process(upstream v6.2 onward), uOps event, sampling period: 100000
89
90 # perf record -e ibs_op/cnt_ctl=1/ -c 100000 -p 1234
91
92Per process(upstream v6.2 onward), uOps event, sampling period: 100000
93
94 # perf record -e ibs_op/cnt_ctl=1/ -c 100000 -- ls
95
96To analyse recorded profile in aggregate mode
97
98 # perf report
99 /* Select a line and press 'a' to drill down at instruction level. */
100
101To go over each sample
102
103 # perf script
104
105Raw dump of IBS registers when profiled with --raw-samples
106
107 # perf report -D
108 /* Look for PERF_RECORD_SAMPLE */
109
110 Example register raw dump:
111
112 ibs_op_ctl: 000002c30006186a MaxCnt 100000 L3MissOnly 0 En 1
113 Val 1 CntCtl 0=cycles CurCnt 707
114 IbsOpRip: ffffffff8204aea7
115 ibs_op_data: 0000010002550001 CompToRetCtr 1 TagToRetCtr 597
116 BrnRet 0 RipInvalid 0 BrnFuse 0 Microcode 1
117 ibs_op_data2: 0000000000000013 RmtNode 1 DataSrc 3=DRAM
118 ibs_op_data3: 0000000031960092 LdOp 0 StOp 1 DcL1TlbMiss 0
119 DcL2TlbMiss 0 DcL1TlbHit2M 1 DcL1TlbHit1G 0 DcL2TlbHit2M 0
120 DcMiss 1 DcMisAcc 0 DcWcMemAcc 0 DcUcMemAcc 0 DcLockedOp 0
121 DcMissNoMabAlloc 0 DcLinAddrValid 1 DcPhyAddrValid 1
122 DcL2TlbHit1G 0 L2Miss 1 SwPf 0 OpMemWidth 32 bytes
123 OpDcMissOpenMemReqs 12 DcMissLat 0 TlbRefillLat 0
124 IbsDCLinAd: ff110008a5398920
125 IbsDCPhysAd: 00000008a5398920
126
127IBS applied in a real world usecase
128
129 ~90% regression was observed in tbench with specific scheduler hint
130 which was counter intuitive. IBS profile of good and bad run captured
131 using perf helped in identifying exact cause of the problem:
132
133 https://lore.kernel.org/r/20220921063638.2489-1-kprateek.nayak@amd.com
134
135IBS Fetch PMU
136~~~~~~~~~~~~~
137
138Similar commands can be used with Fetch PMU as well.
139
140System-wide profile, fetch ops event, sampling period: 100000
141
142 # perf record -e ibs_fetch// -c 100000 -a
143
144System-wide profile, fetch ops event, sampling period: 100000, Random enable
145
146 # perf record -e ibs_fetch/rand_en=1/ -c 100000 -a
147
148 Random enable adds small degree of variability to sample period. This
149 helps in cases like long running loops where PMU is tagging the same
150 instruction over and over because of fixed sample period.
151
152etc.
153
154PERF MEM AND PERF C2C
155---------------------
156
157perf mem is a memory access profiler tool and perf c2c is a shared data
158cacheline analyser tool. Both of them internally uses IBS Op PMU on AMD.
159Below is a simple example of the perf mem tool.
160
161 # perf mem record -c 100000 -- make
162 # perf mem report
163
164A normal perf mem report output will provide detailed memory access profile.
165However, it can also be aggregated based on output fields. For example:
166
167 # perf mem report -F mem,sample,snoop
168 Samples: 3M of event 'ibs_op//', Event count (approx.): 23524876
169 Memory access Samples Snoop
170 N/A 1903343 N/A
171 L1 hit 1056754 N/A
172 L2 hit 75231 N/A
173 L3 hit 9496 HitM
174 L3 hit 2270 N/A
175 RAM hit 8710 N/A
176 Remote node, same socket RAM hit 3241 N/A
177 Remote core, same node Any cache hit 1572 HitM
178 Remote core, same node Any cache hit 514 N/A
179 Remote node, same socket Any cache hit 1216 HitM
180 Remote node, same socket Any cache hit 350 N/A
181 Uncached hit 18 N/A
182
183Please refer to their man page for more detail.
184
185SEE ALSO
186--------
187
188linkperf:perf-record[1], linkperf:perf-script[1], linkperf:perf-report[1],
189linkperf:perf-mem[1], linkperf:perf-c2c[1]